Field of the Invention
The invention relates to a stacked integrated multi-junction solar cell.
Description of the Background Art
Multi-junction solar cells made of different semiconductor materials have been studied for some years in order to achieve the highest possible efficiencies for converting sunlight into electrical energy with solar cells, [W. Guter, Optimization of III-V-based High-Efficiency Solar Cells, Dissertation, University of Constance, Faculty for Physics, 2011]. Multi-junction solar cells divide incident light among a plurality of solar subcells, stacked one above the other, with a different band gap energy. To achieve the highest efficiencies, the semiconductor materials and the band gap energy thereof must be matched to one another, so that each solar subcell, electrically connected in series, generates the same current if possible. Furthermore, apart from high efficiencies, a high radiation stability to high-energy electrons and/or protons (e.g., solar eruption) is also desirable for applications, including space applications.
Provided the semiconductor materials of the individual subcells of the stack have the same lattice constant, the subcells can be manufactured by means of an epitaxial method. A lattice-matched 4-fold AlInGaP/InGaAs/InGaNAs/Ge solar cell is known from Meusel et al., III-V MULTIJUNCTION SOLAR CELLS—FROM CURRENT SPACE AND TERRESTRIAL PRODUCTS TO MODERN CELL ARCHITECTURES, 5th WCPEC, 2010, Valencia, 1AP.1.5. Only insufficient efficiencies are achieved because of crystal quality particularly of the InGaNAs subcell.
If the subcells have different lattice constants, in a first alternative, metamorphic buffer layers are used between two subcells. A sequence of AlInGaP/AlInGaAs/InGaAs subcells, a metamorphic buffer, and Ge is known from Guter et al., DEVELOPMENT, QUALIFICATION AND PRODUCTION OF SPACE SOLAR CELLS WITH 30% EOL EFFICIENCY, European Space Power Conference, 2014, Noordwijkerhout, The Netherlands. Furthermore, another sequence of InGaP/GaAs subcells with a first metamorphic buffer, a first InGaAs subcell and a second metamorphic buffer and a second InGaAs subcell is known from Cornfeld et al., Development of a Four Sub-cell Inverted Metamorphic Multi-junction (IMM) Highly Efficient AM0 Solar Cell, 35th IEEE PVSC, 2010, Honolulu, USA.
A further alternative for combining materials with a different lattice constant is the use of wafer bonding methods. In this case, subcells with different lattice constants are joined. Bonded solar cell stacks with four subcells are known from the dissertation of Uwe Seidel, Interface studies on the tunneling contact of an MOCVD-prepared tandem solar cell, HU Berlin, Mathematical/Natural Science Faculty I, Jan. 9, 2007 and from J. Boisvert et al., Development of advanced space solar cells at spectrolab, in: Photovoltaic Specialists Conference (PVSC), 2010, 35th IEEE, 20-25 Jun. 2010, Honolulu, ISSN: 0160-8371, and from R. Krause et al., Wafer Bonded 4-Junction GaInP/GaAs//GaInAsP/GaInAs, AIP Conference Proceedings 1616, 45 (2014); doi: 10.1063/1.4897025. Further, a 5-fold solar cell stack with two bonded solar cell parts is known from P. T. Chiu et al., Direct Semiconductor Bonded 5J Cell For Space And Terrestrial Applications, IEEE Journal of Photovoltaics, Volume 4(1), pp. 493, 2014.